The invention relates to a battery-powered electrical device comprising a switchable voltage drain and a rechargeable battery. The term "battery" should here be broadly interpreted as encompassing a single cell as well as arrays of such cells (i.e. battery packs).
Devices of this type are well known from everyday experience, and include, for example, electric power tools, video cameras, mini vacuum cleaners, lap-top computers, electronic notebooks, portable music sources, torches, shavers, etc. Many commonly-available battery-powered devices contain a voltage drain (e.g. a motor, lamp, electrical circuit, etc.) which simply continues to draw power from the battery until the battery voltage is no longer sufficient to operate the device; the operator is then expected to recharge the battery before further use. However, it is now generally known that such depletory usage can be detrimental to the performance of the battery in the long run, since discharge of battery cells beyond a certain point can cause the occurrence of irreversible chemical reactions within the cells, which can damage their performance. For this reason, it is generally desirable to interrupt power to the voltage drain well before the battery becomes depleted, and then to recharge the battery before further use.
There are various known ways in which to achieve this object. For example, in International Patent Application WO 96-35252, a resistive strain gauge is attached to an elastic surface in intimate contact with a wall of the battery. As the battery charge drops towards a certain threshold, the occurrence of the chemical reactions referred to in the previous paragraph causes an increase in the battery's internal pressure and/or temperature, resulting in slight expansion of the battery. Such expansion is detected by the strain gauge, which then interrupts the drain of power from the battery.
A disadvantage of the known method and device is that use of the strain gauge relies on the occurrence of chemical reactions which in themselves are a symptom that (slight) over-discharge of the battery has occurred. Thus, although the method prevents severe over-discharge of the battery, it does allow detrimental discharge to occur to some extent.